Field of the Invention
The present invention relates to a lubricating structure for a four-stroke engine, where the lubricating structure lubricates a valve driving mechanism in a cylinder head installed consecutively with a cylinder installed in a crankcase, extending horizontally.
Description of the Related Art
Patent Document 1 (Japanese Patent Laid-Open No. S61-98909) discloses a lubricating structure for a four-stroke engine, in which a cylinder is installed extending horizontally in a crankcase making up a crank chamber, a cylinder head equipped with a valve chamber configured to house a valve driving mechanism is installed consecutively with the cylinder and an oil pan configured to accumulate lubricating oil is provided on a bottom of the crank chamber, wherein the crank chamber and the valve chamber are communicated with each other via a first oil groove and a second oil groove, a check valve configured to permit only flow from the crank chamber to the valve chamber is disposed in the first oil groove, and surplus lubricating oil in the valve chamber is returned (discharged) to the crank chamber by increasing internal pressure in the valve chamber using the check valve.
However, in order to return lubricating oil in the valve chamber to the crank chamber, the lubricating structure for an engine described in Patent Document 1 needs a check valve, which is a dedicated part. Existence of the check valve makes it hard for a mist of the lubricating oil in the crank chamber to flow into the valve chamber, making it necessary to install a dedicated lubricating oil supply line running from the crank chamber to the valve chamber and thereby forcibly lubricate the valve driving mechanism in the valve chamber.